InnoDB with clustered indexes is pretty hard to beat for many scenarios. I hate that Oracle owns it, but damn it works well. Also frequent cascading deletes are surprisingly fast, especially in high lock contention environments. [As a side note, it's unfortunate that Wordpress still uses MyISAM by default] Redis has also been incredibly fast for our ad network. We throw around 7000 qps at the thing with a low number…
After Oracle stopped distributing the source on their website, an open-source project started continuing its development here: http://www.haildb.com/